HuTTON. — On the Gabbro of Dim Mountain. 413 



that these sections must be transverse to the principal axis ; for 

 if not, the cleavages would either be at right angles to each 

 other (Rhombic System, or Ortho-pinacoid), or else the extinc- 

 tion would be oblique to both cleavages (Clino-pinacoid). The 

 edge formed by these cleavage faces is therefore parallel to the 

 prmcipal axis, and as the principal cleavage is not on an axis of 

 elasticity, it must be parallel to one face of the prism : no 

 cleavage seems to be developed parallel to the other face. 



A section, approximately at right angles to the principal 

 axis — as jjroved by the angle between the cleavages — shows, 

 with convergent polarized light, a symmetrical bisectrix with 

 wide axial angle, and the axial plane in the direction of the 

 striations. Cleavage flakes from the principal cleavage (210) 

 give straight extinction, and show an optic axis on the margin 

 of the field, with the axial plane in the direction of the striations ; 

 thus giving a further proof that this cleavage is parallel to the 

 face of the prism. 



Cleavage flakes of the second cleavage show no striations, 

 but extinguish apparently parallel to the first cleavage ; this, 

 however, is not very exactly marked. They show no inter- 

 ference figure with convergent polarized light. 



These straight extinctions, and the bisectrix seen on 001, 

 prove that the mineral belongs to the Rhombic System. Now, 

 in the Rhombic System, the angle between 110 and 100 must 

 lie between 0° and 45°, while the angle between 110 and 010 

 must lie between 45° and 90°. Consequently, as in our case 

 the angle between the two cleavages is about 67°, it follows that 

 the second cleavage, and the plane of the optic axes, are parallel 

 to the brachy-pinacoid. 



The angles of the prism will be 134° and 46°, but the 

 measurements are not very exact, owing to the want of proper 

 instruments ; they are however sufficiently so to show that the 

 mineral is not a rhombic pyroxene but a rhombic amphibole, 

 and probably, therefore, anthophyllite. Pleochroism is well 

 marked in sections more or less parallel to 001. The colour 

 for a being greenish-yellow, and for /3 reddish-brown. Sections 

 parallel to the cleavages do not show any marked pleochroism, 

 so that the colour for y is greenish-yellow, like that for a. 

 Before the blow-pipe the mineral is infusible, or fusible only 

 with great difficulty. All these characters agree with antho- 

 phyllite, but the typical form of that mineral is said by E. S. 

 Dana to have its principal cleavage parallel to 100, and the 

 relative lengths of the lateral axes are not so unequal as in our 

 variety. 



The felspar. — This mineral is so much altered as to show 

 merely a number of granules and rods in a transparent base, 

 which is generally quite amorphous, but occasionally crypto- 

 crystalline. No doubt it is some kind of plagioclase, but 



